Class II Communications

The class II serial data circuit 1807 is used to communicate information between modules (for example the body control module and the engine controller).

Contained in each module's memory is a list of serial data messages that are important and should be received by the module. Also contained are the default actions to be taken by the module, if one of the important messages is not received in time. One of those important messages is the state of health (SOH) message. This message is sent by a module (if no other message needs to be sent) to let all the other modules on the serial data circuit know the sending module is working correctly.

Each time the ignition is turned ON, all the modules on the class II serial data circuit first learn the other modules connected to the class II serial data circuit. Modules are able to learn the other modules on the serial data circuit because each of the important messages has a source identifier as part of the message.

This process of learning by a module allows the module to know if it has lost communication with a specific module on the class II data circuit.